Apostrophe "'s" - is used to show possession & ownership.
Singular Nouns - The general rule is if a noun is singular, and you want to show possession, You add an apostrophe "s" to the end of the word.
If a noun ends in "s", you still just add apostrophe "'s" to the end of the noun in most cases. Their are some exceptions to this rule, but this is the general rule.
My house's roof needs to get fixed.
My computer's very slow.
My boss's car is very nice.
The bus's door is not working very well.
Plural Nouns - that end in "s" just add an apostrophe to the end of the word.
The teachers cars' are really old.
All my teachers' are really nice.
My friends brothers' are really nice to me.
Plural nouns that do not end in s, then just add an apostrophe.
The children's toys are all over the house.
The people's feelings about the issues are very important.
The men's basketball team is doing really well.
Do not use possessive pronouns such as e.g. his, hers, theres, ours etc, with an apostrophe "s". Possessive pronouns are used instead of an apostrophe "s".